In the related art, as a bearing used for a high-speed rotating body, a thrust bearing is known which is disposed so as to face a thrust collar provided on a rotary shaft. As such a thrust bearing, a foil-type thrust bearing, namely a thrust foil bearing is well known. The bearing surface of the thrust foil bearing is formed of a flexible foil (thin metal sheet) in order to accept movement of the rotary shaft (movement in the axial direction of or inclination of the thrust collar) which occurs due to vibration or to impact, and the thrust foil bearing includes a foil structure which is provided under the bearing surface and flexibly supports the bearing surface.
As an example of the thrust foil bearing, a structure is known in which a circular ring-shaped (annular shaped) bearing surface is formed of a plurality of individual foil pieces (top foil pieces) obtained by dividing an annular sheet in the circumferential direction thereof, and a wave sheet-shaped foil piece (bump foil piece) supports each top foil piece (for example, refer to Patent Document 1). Each top foil piece (the thickness thereof is about 100 μm) is arranged at an inclination angle with respect to the thrust collar, and thereby a bearing clearance between the thrust collar and the top foil piece is formed in a wedge shape in side view. That is, the bearing clearance is formed so as to gradually decrease from the leading side toward the trailing side in the rotation direction of the thrust collar (the rotary shaft). Thus, when the thrust collar rotates from the large side (the leading side) toward the small side (the trailing side) of the bearing clearance, a lubricating fluid flows into a narrow part of the wedge-shaped bearing clearance, and the load capability of the thrust bearing is obtained.
Only the edge of the top foil piece on the leading side in the rotation direction of the thrust collar (the rotary shaft) is fixed to a base plate, and the edge is configured as a fixed edge. When a bearing load increases, in a state where the fixed edge (the edge on the leading side) is a fulcrum, the top foil piece moves so as to be horizontal (parallel to the supported surface of the thrust collar), the inclination angle thereof decreases, and at the time the inclination angle becomes about 0.1°, the thrust bearing generates the maximum load capability. In addition, the bump foil piece is arranged so that the ridge line of a peak thereof is parallel to the edge on the trailing side of the top foil piece, and only the edge of the bump foil piece on the trailing side thereof in the rotation direction of the thrust collar (the rotary shaft) is fixed to the base plate. That is, the edge on the leading side of the bump foil piece is set to be a free end.
Since the bump foil piece is arranged and fixed in this way, the pressure of a fluid lubrication film formed at the top foil piece becomes high at the small side (the trailing side) of the bearing clearance. Thus, if this part is supported by a high rigidity, the load capability of the thrust bearing can be increased.
Patent Documents 2 to 4 disclose thrust bearings which include a top foil and a bump foil. Patent Document 5 discloses a thrust bearing which includes a bearing foil, and the bearing surface of the bearing foil is provided with an introducing groove.